The present disclosure is directed to a rate gyro based survey device and a method of conducting a survey of a well borehole. In many instances, a well borehole is drilled which is substantially vertical. Rudimentary survey devices are used for such wells. By contrast, many wells are highly deviated. The well will define a pathway through space which proceeds from a centralized well head, typically clustered with a number of other wells, and extends in a serpentine pathway to a remote point of entry into a producing formation. This is especially the case with offshore platforms. Typically, an offshore platform will be located at a particular location. A first well is drilled to verify the quality of the seismic data. Once a producing formation is located, and is verified by the first well, a number of other wells are drilled from the same location. This is advantageous because it requires that the offshore drilling platform be anchored at a particular location. That is, the offshore drilling platform is anchored at a given site and several wells are then drilled from that site. The wells drilled from a single site will enter the producing formation at a number of scattered locations. As an example, consider a producing formation which is 15,000 feet in length and width and which is located at a depth of 10,000 feet. From a single location approximately near the center, it is not uncommon to drill as many as 30 wells or more to the formation. Consider as an example an offshore location in about 200 feet of water where drilling is conducted into the single formation from a single platform location. After the first well has been drilled, a template is lowered to the mudline and rested on the bottom. The template typically supports several conductor pipes, typically arranged in a grid pattern such as 4.times.8. This provides a template with 32 holes in the template. Conductor pipes are placed in the holes in the template. Below that, a deviated well is drilled for most of the wells. Some of the wells are deviated so that they are drilled at an angle of perhaps only 30.degree. with respect to the horizon as the wells are extended out laterally in a selected direction. The wells enter the formation at predetermined points. This means that each well has a first vertical portion, a bent portion below the conductor pipe, and then a long deviated portion followed by another portion which is often vertical. So to speak, the well is made of serial segments in the borehole.
A survey is necessary to define the precise location of the well borehole. In most deviated wells, a free fall survey instrument typically is not used. Free fall survey instruments are used for fairly vertical wells. Where the vertical component is substantial and the lateral deviation is nil, survey instruments are readily available which can simply be dropped to obtain such data. Alternately, survey instruments are known which can be placed in the drill string at the time of retrieval of the drill string so that well borehole survey data is obtained as the drill string is pulled from the well borehole. This typically occurs when the drill bit is changed. The capture of accurate survey information is important, especially where the well is highly deviated. As an example, the well can be deviated where it extends at a 30.degree. angle with respect to the horizon. It can have two or more large angular deflection areas. The well might terminate at a lateral location as much as 5,000 to 10,000 feet to the side of the drilling platform. Without regard to the lateral extent of the well borehole, and without regard to the azimuth or the depth of the well, it is important to obtain an accurate survey from such wells. In this instance, an accurate survey is required to enable drilling the well to the total depth desired and hitting the target entry into the producing formation. Typically, two or three surveys are required while drilling the well borehole. The surveys that are necessary enable correction to be undertaken so that the well can be further deviated to the intended location for the well.
In one aspect, the present disclosure sets forth a system which is able to be run on a slickline. The slickline is simply a support line to enable the survey sonde to be lowered to the bottom of the well borehole. The borehole path in space is located by the present system. In doing so, the sonde which encloses the equipment of the disclosure is lowered in either of two different fashions. In one instance, it can simply be lowered on the slickline within the drill string, and is then left at the bottom of the drill string, and then is moved incrementally upwardly as the drill string is pulled. Pulling the drill string is necessary in order to change the drill bit which is periodically required. In that sequence, the device is lowered to the bottom of the drill string and is landed just above the drill bit. At that juncture of proceedings, the sonde cannot precede any further because it is captured within the drill string and is too large to pass through the openings in the drill bit. The drill bit is normally replaced by pulling the drill string. The drill string is pulled by removing the topmost joints of pipe. Typically, the derrick is sufficiently tall so that three joints can be removed simultaneously. The three joints together comprise a "stand" which is placed in the derrick to the side of the rotary table. By this approach, the entire drill string is pulled incrementally moving the drill bit toward the surface for replacement. Each stand is approximately 90 feet in height. Therefore the drill bit is stationary for an interval sufficient to remove one stand, and these intervals are spaced at 90 feet in length. At each momentary stop in the process of removing a stand of the drill string, the drill bit is stopped and hence the sonde is stopped and obtains well borehole survey data. As additional stands of pipe are removed, this enables the sonde to stop and to obtain additional well borehole survey data. The data is measured at these stops while the survey is conducted.
In another procedure, the drill string is left in the well borehole. The sonde is lowered inside of the drill string to the bottom of the well borehole on a slickline, and is then pulled from the well borehole. In pulling, measurements are made by periodically stopping the sonde by stopping the slickline movement.
If the slickline remains inside of the drill string during rotation in the drilling phase, it can be readily severed. A line cutting device is available which can be placed on the slickline and which is permitted to fall to the bottom of the slickline. The inertial upset which occurs when the cutting device strikes bottom is sufficient to cut the slickline and thereby to enable retrieval of the slickline cutting apparatus and the slickline prior to resuming the drilling phase. This leaves the sonde in the drill pipe. It is left so that it can be retrieved along with the drill string. It is always found in the last joint of the drill stem (normally the bottom most drill collar) which is removed at the time that the drill string is pulled. As mentioned, pulling normally occurs during a trip to replace the drill bit.
The present disclosure sets forth an apparatus which particularly has an advantage in overcoming modest amounts of instrument drift. It utilizes a rate gyro as well as two accelerometers. Both devices provide measurements in orthogonal directions. In the preferred construction of the device, measurements are made in the X and Y dimensions. By definition, the Z dimension is coincident with the center line axis of the cylindrical sonde. Therefore X and Y define a plane at right angles with respect to the Z axis. There is a scale problem which arises from the use of a rate gyro mixed with accelerometers. The sensitivity of a gyro is enhanced compared with accelerometers. Typically, the signals from the rate gyro are approximately two orders of magnitude more sensitive. This means that instrument drift resulting from aging drift, temperature drift, drift as a result of vibration and the like are substantially amplified in the output signals from the rate gyro. One advantage of using a rate gyro is that the signal is so sensitive. It is however a detriment if the rate gyro signal is to be used in conjunction with signals from accelerometers. The present disclosure sets forth a mechanism in which the enhanced sensitivity of the rate gyro compared with the accelerometers is used to an advantage. One aspect of this derives from a mechanism which rotates the rate gyro housing 180.degree.. The housing is coincident with the axis through the tool so that the rate gyro is rotated about the Z axis. If the rotation is precisely 180.degree., then the X and Y outputs from the rate gyro will be reversed. They will be reversed precisely thereby yielding the same output data with a reversal in algebraic sign. If a value is obtained denoted as +X, and a second value is obtained which is denoted as -X, then the algebraic sum of these two values should be zero in a perfect situation where no systematic error such as instrument drift occurs. Should there be a minor amount of error in the system such as drift or other error, the magnitude of the algebraic sum of these two values is dependent on the error, and more precisely is two times the error. This will be represented below as 2.DELTA.. Knowing this, the error .DELTA. can be isolated, and can then be eliminated from the data. Not only is this is true for the X dimension, it is also true for the Y dimension. Therefore both errors in X and Y can be overcome. This enables the presentation then of a rate gyro signal which is substantially free of that type of error.
The present disclosure takes advantage of onboard computing through a CPU which is provided with suitable power for operation by a power supply, and which works with data which is input to the CPU. The data from the rate gyro and the two accelerometers is written temporarily in memory. After a set of data is obtained, the set is then processed to reduce the amount of memory storage required. Speaking more specifically, in one aspect of the present disclosure, a set or ensemble of data is obtained. The number of measurements from each sensor output is represented by N where N is a positive integer. The integer is typically a multiple of two so that data processing is simplified. In one aspect of the present disclosure, N is typically 64, 128, 256, . . . . As will be seen, these represent values of N, where N is a multiple of two.
In summary, the present disclosure sets fourth a method and apparatus for obtaining survey data from a slickline supported tool which is maintained on the slickline or which is left in the drill string just above the drill bit. In both aspects, data is taken as the sonde which encloses the apparatus is pulled toward the surface, either on the slickline or on removal of the drill string from the well borehole. In both instances, data is captured by making multiple measurements at a given depth in the well borehole whereby N data from each sensor output are collected and processed. The data are obtained from X and Y accelerometers and X and Y output sensors on a rate gyro. This provides four sets of data. The data are stored temporarily in memory until the N data measurements are accumulated from each of the four sensor outputs. The sensors provide this data at one position, and then the rate gyro housing is rotated so that the data is provided from an alternate position. The alternate position is intended to be precisely equal and opposite. The second set of N data therefore provides data which ideally should subtract from the first set of data for the rate gyro. The N data are then averaged to provide four average values for each rate gyro orientation, two of which derived from the rate gyro and two of which are obtained from the accelerometers. This enables nulling to substantially reduce the highly amplified effects of drift and the error in the rate gyro data. The several data for each of the four sensors are statistically analyzed to provide the standard deviation. This is an indication of data quality.